The invention relates to a process for preparing isocyanates by reacting the corresponding amines with phosgene in the liquid phase, if appropriate in the presence of at least one inert medium, in which the amine and the phosgene are first mixed in a mixing chamber to give a reaction mixture and the reaction mixture is fed to a reactor. The amine is added through an orifice arranged coaxially to the mixing chamber and the phosgene is added through feed orifices in at least two planes arranged at right angles to the axis of the mixing chamber. At least one plane is arranged upstream and one plane downstream of the orifice for addition of the amine in main flow direction of the reaction mixture.
Isocyanates can be prepared by phosgenating the corresponding amines, in principle by a liquid phase or gas phase phosgenation. Liquid phase phosgenation is notable in that the reaction can be carried out at lower temperatures than the gas phase phosgenation and no evaporation of the reactants is required.
In liquid phase phosgenation, an amine-containing reactant stream is fed in in the liquid phase. This is mixed with a phosgene-containing reactant stream. The phosgene may be dissolved in an inert solvent. Subsequently, the phosgene-containing reactant stream is sprayed into a mixing chamber in which it is mixed with the amine-containing reactant stream. The amine and the phosgene react with release of HCl to give the corresponding isocyanates.
Rapid mixing of the amine with the phosgene is necessary, since the isocyanate formed, in the case of too low a phosgene concentration, reacts with the excess amine to give urea or other troublesome, high-viscosity and solid by-products. For this reason, rapid mixing and a short residence time in the reaction chamber are required.
An apparatus in which the amine and the phosgene are first mixed in a mixing chamber to give a reaction mixture and the reaction mixture is then fed to a reactor, the amine being added through an orifice arranged coaxially to the mixing chamber and the phosgene being added through feed orifices in at least two planes arranged at right angles to the axis of the mixing chamber, is described, for example, in DD-A 300 168.
It is an object of the present invention to provide a process for preparing isocyanates by reacting the corresponding amines with phosgene in the liquid phase, in which a lower level of secondary component formation can be achieved compared with the processes known from the prior art.